Use of a chromium steel as raw material for corrosion-resistant spring elements and method for producing said chrome steel

ABSTRACT

A ferritic chromium steel comprising 0.03 to 0.1% of carbon, 0.2 to 0.9% of silicon, 0.3 to 1% of manganese, 13 to 20% of chromium, up to 0.5% of nickel, 0.1 to 1.5% of molybdenum, 0.1 to 0.5% of copper, 0.03 to 0.05% of nitrogen, less than 10 ppm of boron, up to 0.01% of titanium, 0.01 to 0.10% of niobium, 0.02 to 0.25% of vanadium and up to 0.002% of aluminum, remainder iron, is distinguished by a high corrosion resistance and is suitable as a material for cold-formed spring elements with improved spring properties and a high dimensional accuracy, in particular for leaf springs, spring rails for windscreen wipers and reed lamellae for textile machines, oil stripper rings for internal combustion engines and sealing lamellae for hydraulic installations.

The invention relates to the use of a chromium steel comprising 0.03 to 0.1% of carbon, 0.2 to 0.9% of silicon, 0.3 to 1% of manganese, 13 to 20% of chromium, less than 0.5% of nickel, 0.1 to 1.5% of molybdenum, 0.1 to 0.5% of copper, 0.03 to 0.05% of nitrogen, less than 10 ppm of boron, less than 0.01% of titanium, 0.01 to 0.10% of niobium, 0.02 to 0.25% of vanadium and preferably less than 0.002% of aluminum, remainder iron.

Chromium steels with a ferritic or, depending on their nickel content, a ferritic-austenitic microstructure have a high resistance to corrosion and are known in numerous forms.

For example, European laid-open specification 1 099 773 A1 describes a ferritic chromium steel containing 0.02 to 0.06% of carbon, up to 1% of silicon, up to 1% of manganese, 11 to 30% of chromium, up to 0.7% of nickel, up to 0.05% of phosphorus, up to 0.01% of sulfur, up to 0.005% of aluminum, and the nitrogen, vanadium and carbon contents of which are matched to one another in a specific way. This steel is very soft and is therefore suitable as a material for stainless deep-drawing sheets with a low anisotropy.

Furthermore, European laid-open specification 1 113 084 A1 describes a ferritic chromium steel containing up to 0.1% of carbon, in each case up to 1.5% of silicon and manganese, 5 to 50% of chromium, up to 2% of nickel, up to 2.5% of molybdenum, up to 2.5% of copper, up to 0.1% of nitrogen, up to 0.05% of boron, up to 0.5% of titanium, up to 0.5% of niobium, up to 0.3% of vanadium, up to 0.08% of phosphorus, up to 0.02% of sulfur, up to 0.2% of aluminum, up to 0.3% of zirconium and up to 2.0% of tungsten. The steel is very soft and, on account of its good deformability, is likewise suitable as a material for deep-drawing sheets with a defined crystal orientation following rolling deformation and a final anneal at 800 to 1100° C.

Finally, U.S. Pat. No. 5,178,693 describes a ferritic-austenitic chromium steel containing 0.01 to 0.15% of carbon, up to 1.39% of silicon, 0.1 to 4.0% of manganese, 10 to 20% of chromium, up to 2.5% of molybdenum, 0.1 to 4.0% of copper, up to 0.032% of nitrogen, up to 0.0050% of boron, up to 0.02% of vanadium and up to 0.20% of aluminum. This steel is suitable for the production of thin strip, from which, following a final continuous anneal at 300 to 650° C., leaf springs can be produced by stamping or cutting them out. However, it has emerged that the strip, after the final anneal at the temperature indicated and cooling to room temperature, has high internal stresses which, depending on its thickness, leads to distortion of the stamped parts when they are being stamped out. This is a significant drawback in the case of spring elements, in particular in the case of lamellar springs, since they then still have to be straightened and bent for final shaping. Moreover, if the internal stresses are relatively high, the fatigue strength may drop.

Materials used for spring elements require a high linear spring rate, which is determined from the inclination of a force/deflection straight line using the formula R=δF/δs

-   -   where F is the force and s is the deflection. Furthermore, in         the case of spring elements the permissible spring limit stress         is a characteristic feature which is calculated, in accordance         with DIN 2088 and DIN 2089, from the tensile strength R_(m)         multiplied by a constant. Depending on the spring geometry and         application, this constant is from 0.4 to 0.7 depending on the         individual circumstances.

Although carbon steels with carbon contents of up to 1% have high R_(m) values after a heat treatment and therefore also allow high limit stresses, their corrosion resistance, and in particular their resistance to rust, is low. This is a serious drawback, since spring elements are generally exposed to humid air and are also in widespread use in the chemical industry. Consequently, austenitic steels are more suitable, on account of their high corrosion resistance. However, these steels require a relatively high level of expensive alloying elements, such as in particular nickel. The same is also true of precipitation-hardenable austenitic steels, which contain aluminum, titanium or niobium in order to improve their strength.

Moreover, for health reasons, nickel-containing steels are not suitable for objects which come into contact with the human skin, for example jewellery.

Martensitic chromium steels are less expensive but have the drawback of poorer cold-workability, and can therefore only be processed to form spring elements in the annealed state. The springs produced therefrom require an anneal or hardening treatment at high temperatures and subsequent tempering. The hardening of the individual parts is associated with high costs and, for reasons of quality, requires a hardness test for final inspection, and consequently the cost benefit derived from the elimination of expensive alloying elements is lost again. Moreover, in the case of martensitic chromium steels there is a risk of islands and nests of chromium carbides forming in the microstructure, leading to a deterioration in the corrosion resistance.

Even a coating is unable to achieve an extensive, in particular prolonged improvement in the corrosion resistance, since coatings of this type have to have a very low thickness and lose their protective action as a result of wear or damage. Moreover, in the case of metallic coatings, local elements which lead to increased corrosion may form at damaged locations. The risk is particularly high in the case of spring elements, since such elements are often provided with stamped-out portions and spot welds or are arranged in metallic clamping guides in order for their position to be fixed.

The invention is aimed at improving the spring properties, characterized by the tensile strength and dimensional accuracy of spring elements made from a corrosion-resistant steel.

To achieve this, the invention proposes the use of a ferritic chromium steel comprising 0.03 to 0.1% of carbon, 0.2 to 0.9% of silicon, 0.3 to 1% of manganese, 13 to 20% of chromium, less than 0.5% of nickel, 0.1 to 1.5% of molybdenum, 0.1 to 0.5% of copper, 0.03 to 0.05% of nitrogen, less than 10 ppm of boron, less than 0.01% of titanium, 0.01 to 0.10% of niobium, 0.02 to 0.25% of vanadium, remainder iron.

A ferritic chromium steel comprising 0.03 to 0.08% of carbon, 0.2 to 0.9% of silicon, 0.4 to 0.8% of manganese, 15 to 18% of chromium, less than 0.2% of nickel, in each case 0.1 to 0.5% of molybdenum and copper, 0.03 to 0.05% of nitrogen, less than 8 ppm of boron, less than 0.005% of titanium, 0.01 to 0.05% of niobium and 0.05 to 0.20% of vanadium, remainder iron, is particularly suitable.

The invention combines the high corrosion resistance of the ferritic chromium steels with the high tensile strength of high-alloy spring steels; it makes use of the following discoveries:

Higher annealing temperatures, in particular annealing temperatures of from 1000 to 1200° C., can be used as a result of the titanium content being reduced to less than 0.005%. Under these conditions, no titanium carbides and/or titanium carbonitrides are formed as MX precipitations which have an embrittling action. These compounds would preferentially form at the grain boundaries, thereby preventing or making more difficult subsequent cold-working.

On account of the higher annealing temperature, it is possible to increase the dissolution of the carbides and/or carbonitrides to such an extent that—on account of the absence of precipitation nuclei of titanium carbide or titanium carbonitride—after quenching a very high proportion of alloying elements remains metastable in solution. This higher level of dissolved or metastable-dissolved elements and/or precipitations ought to be responsible for the excellent cold-workability and for the high work hardening of the steel in accordance with the invention.

Furthermore, the limiting of the titanium compound combined, at the same time, with micro-alloying with the elements vanadium and niobium, particularly advantageously prevents titanium-containing MX precipitations acting as isomorphic nuclei, i.e. nuclei with an identical lattice structure, for common, coherent vanadium and niobium precipitations. Since vanadium is preferentially precipitated in nitride form, whereas niobium is preferentially precipitated as 50:50 carbonitride, the growth kinetics of these precipitations are different. The increase in strength resulting from a heat treatment at temperatures from 100 to 300° C. is attributable to the growth of metastable precipitations.

To avoid relatively insoluble borides, the boron content should be below 10 ppm, and the aluminum content should be below 0.002%.

Furthermore, the steel may also contain less than 0.002% of aluminum.

In the proposed steel, the carbon and nitrogen and/or niobium, vanadium and titanium contents are preferably matched to one another as follows:

-   -   (% C)/(% N)=0.8 to 2.0     -   [(% Nb)+(% V)]/10(% Ti)=5 to 17.

The steel according to the invention is distinguished by an extraordinarily high tensile strength, an excellent cold-formability and high corrosion resistance; it has a very fine-grade microstructure and allows high temperatures to be used in a solution anneal without the risk of grain boundary embrittlement as a result of carbides and/or carbonitrides. After a solution anneal of this type, which is a typical for ferritic steels, preferably for one to fifteen minutes at 1000 to 1200° C., these compounds remain metastable in solid solution and allow optionally multistage cold-working. The cold-working is preferably followed by final heating at 100 to 400° C., preferably at most 300° C., for ten to fifteen minutes.

In detail, the chromium steel according to the invention may be cold-worked in the form of round wire with a decreasing cross section of up to 40%, preferably up to 35%, and a solution anneal may then be carried out in order to substantially eliminate the carbide and carbonitride precipitations, with subsequent quenching. When it is in the state in which it has been cold-worked, solution-annealed and quenched, the steel or wire has an excellent cold-workability, which can be improved still further by a further cold-working with decreases in cross section of up to 65%, for example 50%. The strength of the steel then already exceeds that of a conventional cold-rolled spring steel strip in accordance with DIN 17 222 and DIN 17 224, with R_(m) values of from 1150 to 1500 N/mm² for steel grades Ck 55, Ck 67, Ck 101 and 50CrV4.

The microstructure of the heat-treated and cold-worked steel according to the invention, with a grain size of less than 20 μm, is extraordinarily fine-grained, as is clear from the microstructure image shown in FIG. 2, compared to the starting microstructure, which is already fine-grained, shown in FIG. 1. The microstructure image shown in FIG. 2 reveals that the microstructure is ferritic but contains a small quantity of transformation microstructure constituents (dark areas), which must be martensite, which has the effect of increasing strength.

Finally, the steel may also be subjected to a third cold-working with a degree of deformation of up to 12%, in which a wire with a rectangular cross section is produced from the round wire in order to reduce the grain size to less than 15 μm.

Irrespective of the number of deformation stages, the steel should be age-hardened at a low temperature, preferably at 100 to 400° C., more preferably at at most 300° C., in order to further increase the tensile strength.

This age-hardening at a very low temperature is preferably carried out under the action of a stress and/or a surface pressure of from 20 to 100 N/mm² and serves to eliminate any internal stresses, in particular transverse stresses.

The steel is particularly suitable for use as a material for producing leaf springs, spring rails for windscreen wipers, reed lamellae for textile machines, oil stripper rings for internal combustion engines and sealing lamellae for hydraulic installations. On account of its low nickel content, moreover, the steel has a very good compatibility with the skin and is therefore also suitable as a material for bracelet and strap clasps, bracelets and straps and items of use with a low nickel release rate in accordance with EU directive 94/27 EC dated Jun. 30, 1994 (cf. AJ L 188/1), which stipulates a release rate of less than 0.5 μg/cm²/week, whereas in the case of a conventional 18/8 chromium nickel steel the nickel release rate may reach up to 100 μg/cm²/week.

The steel according to the invention has a corrosion resistance and spring properties which, measured on the basis of the tensile strength, reaches the level of high-alloy austenitic steels, such as X5CrNiMo18,10.

The steel has a ferritic microstructure with niobium and/or vanadium carbides or niobium carbides; however, on account of its titanium content of less than 0.01%, preferably less than 0.006%, it does not contain any titanium-containing precipitations. Specifically, tests have revealed that the titanium carbides are retained during the solution anneal and are not dissolved. To this extent, the titanium carbides behave differently than the carbides of vanadium and niobium, which dissolve. Moreover, the titanium carbides cause grain boundary precipitations which have an embrittling action at high annealing temperatures, and consequently the titanium content should be below 0.01%, preferably below 0.006%, even better below 0.004%.

The invention is illustrated by way of example in the block diagram presented in FIG. 3; it is explained in more detail below on the basis of comparative tests.

EXAMPLE 1

A round wire made from steel A1 in accordance with Table I with a diameter d₀ was rolled down to a diameter d by means of driven hard-metal disks. The degree of deformation was calculated for each test as a relative dimension change E using the formula ε=100·Δd/d ₀ from the cross-section difference Δd=d₀−d.

In each series of tests, the adjustment of the hard-metal disks was altered toward an increasing reduction in cross section, until surface defects, in particular surface cracks, occurred or the adjustment forces or the rolling forces acting on the hard-metal disks reached a predetermined limit level.

The degrees of deformation are summarized in Table II, in which ε₁, ε₂ and ε₃ denote the degrees of deformation of the first, second and third cold-working steps.

The round wire which had been cold-worked with degree of deformation ε₁ was heated in a continuous annealing furnace under shielding gas with a dew point below −65° C. to the temperature T1 shown in Table II. On leaving the heating zone of the furnace, cold shielding gas was flushed around the solution-annealed round wire in order to avoid oxidation, and the wire was then quenched with water and dried in air.

During a final heat treatment following a cold-working operation, the round wire made from steel A1 was subjected to a final anneal in a continuous process in a furnace provided with driven rolls at both the entry side and the exit side. It was in this way possible to heat the round wire under tensile stress in a heating tube, with the aid of infrared rays, at the temperatures T2 shown in Table II. The rotational speeds of the driven rolls were controlled in such a way during the heating that the round wire was under a tensile stress of 20 N/mm² and a heat treatment of 35 min resulted from the drawing speed.

The round wire which had been heat-treated in this way was processed to form spring elements. Investigations revealed only a slight scatter in the spring properties.

EXAMPLE 2

To produce compression and/or oil stripper rings or piston rings for internal combustion engines, a round wire made from the steel A2 in accordance with Table I was firstly deformed with a degree of deformation of ε₁=23% to form a flattened wire with a square cross section. The flattened wire was then heated under shielding gas in a continuous process in a heating furnace to 1065° C. and then quenched in water. After intermediate drying, the wire was deformed with the aid of a cartridge rolling device with a degree of deformation of ε₂=43% to form a preliminary profiled section. This was followed by further working with the aid of a drawing die with a degree of deformation of ε₃=6% to produce the predetermined piston ring cross section.

The finished wire had a tensile strength of 1620 N/mm² with a residual elongation of 3%.

The second heat treatment (final heating) is not absolutely imperative, since, for example, piston rings with a slight ovality of up to a few μm, in the installed state, on account of their ovality are under a force fit or mechanical stresses, but these are quickly broken down after the engine has started to be used as a result of the combustion heat which occurs.

Two conventional chromium steels B1 and B2 with a composition which meant that the carbon was stably bonded by titanium as titanium carbide are compared with the steels A1 to A3 according to the invention in Table I. The data in Table II reveals that in the case of these comparison steels, the maximum permissible annealing temperature of 800° C. from the specialist literature relating to ferritic chromium steels must not be exceeded, since otherwise grain boundary embrittlement occurs, making subsequent cold-working extremely difficult or even impossible. By contrast, the chromium steels according to the invention, as shown by the data for the test steels A1 to A3, can be annealed at considerably higher temperatures and accordingly have better cold-working properties and in particular an advantageous performance during low-temperature final annealing. This is true in particular if the titanium, niobium and vanadium and/or carbon and nitrogen contents are matched to one another in accordance with the invention.

It is clear from the data given in Table I that the steels according to the invention A1 to A3 reach a tensile strength (Rm1) of up to 1590 N/mm² as the temperature T1 for the first heat treatment and a subsequent cold-working increases. At lower temperatures of, for example, 850° C., it is impossible to achieve any significant increase in strength even after cold-working, as revealed by the data from the two tests 1 and 2. Tests 14 to 16 for the conventional steel B1 have the same characteristics. This means that the temperature used during the solution annealing should be over 850° C. and should preferably be 1000 to 1200° C.

The data for tests 3 to 13 using the chromium steels A1 to A3 according to the invention reveal the importance of a sufficiently high temperature during the first heat treatment in conjunction with cold-working in accordance with the invention and, furthermore, demonstrate how much the tensile strength can be increased with the aid of the second heat treatment at a temperature of up to 300° C. In this respect, test 10 reveals that a heat treatment at 350° C. is no longer associated with an increase in strength.

The high strength values demonstrated by the data of tests 1 to 13 for the steels according to the invention can be attributed to microstructure precipitations which result during the cold-working and heating in accordance with the invention (cf. FIG. 2). This applies in particular to the quenching from the high temperature of the first heat treatment (solution anneal). It is particularly noticeable in this context that the solution anneal (at temperatures from 1000° to 1200° C.) at high temperatures is not associated with grain boundary embrittlement caused by carbides and/or carbonitrides.

The process steps according to the invention, by contrast, in the comparison steels B1 and B2 do not lead to any significant improvement in the tensile strength, as shown in tests 14 to 23. Although cold-working with degrees of deformation of up to 40% is still possible after a first heat treatment at 850° C. (tests 15 and 16), this does not involve any significant increase in strength. At higher annealing temperatures of over 1000° C., by contrast, grain boundary embrittlement, which is typical of ferritic chromium steels, occurs, making subsequent cold-working impossible. TABLE I Alloy Cr Si Mn C N Ni Mo Ti Nb V Cu B No. Invention (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (ppm) A1 Yes 16.6 0.5 0.49 0.07 0.048 0.39 0.12 0.002 0.16 0.14 0.11 3 A2 Yes 18.3 0.42 0.63 0.05 0.052 0.25 0.25 0.001 0.15 0.20 0.21 5 A3 Yes 18.7 0.46 0.63 0.035 0.038 0.21 0.32 0.001 0.17 0.10 0.31 5 B1 No 18.0 0.52 0.38 0.043 0.021 0.52 0.02 0.18 0.04 0.06 0.12 15 B2 No 18.7 0.61 0.52 0.081 0.029 0.46 0.06 0.12 0.06 0.09 0.34 12

TABLE II Alloy Test ε1 T1 ε2 ε3 Rm1 T2 Rm2 No. No. (%) (° C.) (%) (%) (N/mm²) Deformability (° C.) (N/mm²) A1 1 10 850 30 none 680 good none — A1 2 10 850 30 15 750 good 120 745 A1 3 10 1050 40 15 1175 good 120 1220 A1 4 35 1050 30 10 1425 good none 1485 A1 5 35 1050 40 10 1480 good 120 1560 A2 6 35 1050 50 15 1495 good 150 1650 A2 7 35 1100 50 10 1540 good 150 1695 A3 8 35 1120 40 15 1515 good none — A3 9 35 1120 50 15 1535 good 100 1620 A3 10 35 1120 60 15 1550 good 150 1680 A3 11 35 1120 70 15 1590 good 250 1710 A3 12 35 1120 70 15 1590 good 350 1490 A3 13 35 1120 70 15 1590 good 450 1405 B1 14 10 850 15 15 690 good 150 690 B1 15 25 850 30 15 830 good 150 835 B1 16 25 850 40 — 1150 poor 150 1145 B1 17 25 1000 10 — 990 poor 150 995 B1 18 35 1050 — — — very poor — — B2 19 35 1000 10 — 620 poor 150 630 B2 20 35 1000 18 — 675 poor 150 670 B2 21 25 1050 — — — poor — — B2 22 35 1050 — — — poor — — B2 23 25 1100 — — — very poor — — 

1. The use of a ferritic chromium steel comprising 0.03 to 0.1% of carbon, 0.2 to 0.9% of silicon, 0.3 to 1% of manganese, 13 to 20% of chromium, less than 0.5% of nickel, 0.1 to 1.5% of molybdenum, 0.1 to 0.5% of copper, 0.03 to 0.05% of nitrogen, less than 10 ppm of boron, less than 0.01% of titanium, 0.01 to 0.10% of niobium, 0.02 to 0.25% of vanadium, less than 0.002% of aluminum, remainder iron as a material for corrosion-resistant spring elements.
 2. The use of a chromium steel as claimed in claim 1, which contains less than 10 ppm of boron and/or less than 0.002% of aluminum.
 3. The use of a steel as claimed in claim 1, characterized in that the carbon and nitrogen contents satisfy the condition (% C)/(% N)=0.8 to 2.0.
 4. The use of a chromium steel as claimed in claim 1, characterized in that the niobium, vanadium and titanium contents satisfy the condition [(% Nb)+(% V)]/10(% Ti)=5 to
 17. 5. The use of a chromium steel as claimed in claim 1 in the state in which it has been solution-annealed, cold-worked and tempered at low temperatures.
 6. The use of a chromium steel as claimed in claim 1 for producing dimensionally stable, low-distortion objects by stamping or cutting.
 7. The use of a chromium steel as claimed in claim 1 as a material for leaf springs, spring rails for windscreen wipers, piston rings for internal combustion engines, sealing lamellae for hydraulic installations, reed lamellae and for products which come into contact with the skin.
 8. A process for improving the spring properties of material in strand form, in which a ferritic chromium steel comprising 0.03 to 0.1% of carbon, 0.2 to 0.9% of silicon, 0.3 to 1% of manganese, 13 to 20% of chromium, less than 0.5% of nickel, 0.1 to 1.5% of molybdenum, 0.1 to 0.5% of copper, 0.03 to 0.05% of nitrogen, less than 10 ppm of boron, less than 0.01% of titanium, 0.01 to 0.10% of niobium, 0.02 to 0.25% of vanadium, less than 0.002% of aluminum, remainder iron is cold-worked to a degree of deformation of up to 40%, then solution-annealed and quenched.
 9. The process as claimed in claim 8, characterized by solution annealing at 1000° to 1200° C.
 10. The process as claimed in claim 8, characterized in that the solution-annealed steel is cold-worked with a degree of deformation of up to 65%.
 11. The process as claimed in claim 10, characterized in that the cold-worked steel is hot age-hardened at a temperature of from 100° to 400° C.
 12. The process as claimed in claim 10, characterized in that the steel with a degree of deformation of up to 12% is set to a mean grain size of less than 15 μm.
 13. The process as claimed in claim 11, characterized by a final anneal under stress.
 14. The process as claimed in claim 13, characterized by a tensile stress of from 20 to 100 N/mm². 